High-speed optical networks transmit information long distances as light signals through optical fiber networks. Amplification of optical signals, preferably using optical amplifiers, is required at regular intervals along a long distance network to maintain signal strength. Since some advanced networking techniques transmit many channels simultaneously through the same fiber, it is important that the gain is the same for each channel. For example, networks that use wavelength division multiplexing (WDM) transmit many channels over the same fiber, with each channel at a different wavelength. Since current optical amplifiers have wavelength dependent gains, repeated amplification can distort the information being transmitted. Wavelength-dependent gain can be overcome by optical equalization of the different signal channels. In practice, this equalization is performed by use of a variable optical attenuator (VOA).
There are several types of VOAs currently available. In one type of VOA, a Mach-Zehnder interferometer has a material in the optical path having a temperature dependent refractive index (the “thermo-optic effect”). The interferometer is configured such that a change in the temperature of the thermo-optic material results in a change in the output light intensity. Attenuation is thus adjusted by control of the temperature of the thermo-optic material. Although thermo-optic VOAs have good optical coupling properties and are polarization independent, they suffer from high power usage and a relatively slow response time of greater than 10 ms, and thus they are not appropriate for high speed networking.
Another currently available VOA employs micro-electro-mechanical system (MEMS) elements, in which movable, micro-elements are used to attenuate light. MEMS devices are also relatively slow, with response times on the order of milliseconds, and have reliability issues resulting from the many moving parts of the VOA.
In addition to speed and reliability, there is also a need to have VOAs that can be assembled into arrays or into other devices. For the large number of channels envisioned for WDM networks, it would be a great advantage to be able to fabricate VOAs of smaller size, to assemble VOAs into arrays, and to incorporate them into other WDM devices, such as multiplexers or demultiplexers. Heretofore, it has been difficult to configure VOAs into arrays or within other devices.
Therefore, it would be desirable to have a VOA that is faster than currently available devices, and that can be assembled in large numbers as an array. It is also desirable to have a VOA that is manufactured by techniques that allow for integration into other WDM devices.